The militray services use high power VLF (very low frequency) transmitters for reliable long range communications. Very low frequencies of the radio spectrum are customarily utilized for such transmitters with frequency shift keying as a common mode of transmission, because ground waves in which the energy is transmitted are only slightly subject to fading and to daily and seasonably variations.
However, the antennas which have been commonly used often have such a narrow bandwidth that during keying, the transmitter frequency is so far off antenna resonance that radiated power is reduced and the transmitter is operated inefficiently into a highly reactive load. One effective method of overcoming the foregoing problem has been to dynamically tune the antenna in synchronism with the transmitter frequency. A conventional technique for dynamic tuning of the antenna has been to inductively couple a number of inductance coils to part of a coil comprising the antenna tuner circuit and thus form what is known to those skilled in the art as a Tuner Coil. By shorting the coupled inductance coils with electronic switching devices, the Tuner Coil inductance has been changed by a single fixed amount from L to L-.DELTA. L, and as a result the antenna resonant frequency may be changed from f to F+.DELTA. f. However, for practical purposes the antenna system inductance decreases with increasing frequency and the same .DELTA. L has been known to produce a larger .DELTA. f in the antenna resonant frequency at higher frequencies. As a result, the frequency range is limited for which a conventional Tuner Coil could present an acceptable load phase angle to a transmitter. Extending the frequency range necessitated additionally switching one or more relatively large, space consuming coils in shunt across the Tuner Coil. The shunt coil can cause the same Tuner Coil .DELTA. L of the antenna system to make a lesser .DELTA. f change in the antenna resonant frequency at higher frequency ranges by effectively reducing the value of inductance of the tuner circuit.
Other examples of prior art antenna tuning devices include U.S. Pat. No. 2,989,624 to Mark I. Jacob, issued June 20, 1961 and U.S. Pat. No. 3,319,168 to Wayne R. Olson, issued May 9, 1967, both patents being assigned to the present assignee. U.S. Pat. No. 2,989,624, which discloses apparatus for resonantly tuning an antenna system, utilizes thyratron tube switches to short circuit a plurality of normally open circuited inductance coils to vary the amount of inductance reflected into the antenna resonant load circuit. The disadvantages of employing tube switches include the amount of space consumed by the switches, the heat generated and the relative slowness in which the switches can respond to the rapid keying rate in order to short circuit the normally open circuit inductance coils. In U.S. Pat. No. 3,319,168, this problem is remedied by the use of pairs of Silicon Controlled Rectifiers as switching components. However, both of the aforementioned patents include the inherent shortcoming of changing the center tuning operating frequency of an antenna by utilizing the same, single tuning coil or serially connected tapped segments thereof. Utilizing the same tuning coil or segments thereof has been found to be impractical, because if the system were employed with a relatively high powered transmitter, conventional Silicon Controlled Rectifiers cannot efficiently sustain the resulting power load thereon as a consequence of the present coupling arrangements of the shorted inductance coils to the single coil comprising the conventional tuner circuit.
Accordingly, it is an object of the instant invention to provide improved arrangements for changing the resonant frequency of a very narrow band antenna to compensate for deviations in operating frequency of the transmitter and at a rate in synchronism with the transmitter frequency. It is also an object of the invention to set the deviation of antenna resonant frequency to correspond to the transmitter frequency deviation by the utilization of a unique Tuner Coil arrangement connected in series with the main antenna tuning coil. It is a further object of the instant invention to provide a dynamic antenna tuner which uses the same inductively coupled normally open circuited inductance coils to obtain several different values of inductance change in a relatively small amount of space and without necessitating the use of a conventional space consuming shunt coil. It is a still further object of the invention to employ a Tuner Coil of unique configuration which can have desired self and mutual inductances and proper inductive coupling within a minimum of space.